Electrical logging refers generally to the surveying of oil or gas wells to determine their geological, petro-physical, or geophysical properties using electronic measuring instruments. The electronic measuring instruments are conveyed into a wellbore with an electrical cable (e.g., an armored steel cable), often referred to as a wireline cable. Measurements made by downhole instruments secured to the wireline cable are transmitted back to a data processing system located at the earth surface through electrical conductors in the wireline cable. Electrical, acoustical, nuclear and imaging tools are used to stimulate the formations and fluids within the wellbore and the electronic measuring instruments then measure the response of the formations and fluids. The wireline cable provides also the electrical power required by the logging tools to operate.
From an electric power perspective, a wireline logging system may be viewed as an electrical circuit having a head resistance (Rh) representing the downhole instruments in series with an impedance element representing the wireline cable itself. If the wireline cable is modeled as a simple resistive element, FIG. 1 illustrates a circuit diagram of the system. In this simple model, a voltage Vs from a voltage source is applied across the series combination of the cable resistance (Rc) and the head resistance Rh.
Two competing considerations govern wireline logging operations. On the one hand, it is desirable to maximize the power delivered to the head (tools) to maximize the rate of data acquisition and to speed up operations in general. Maximum power is delivered to the load when the cable resistance Rc equals the head resistance Rh and the voltage at the head Vh is half the source voltage Vs. Under these conditions, however, the load voltage Vh varies by approximately 100% when the load impedance (Rh) goes from this value to almost open circuit, which occurs when heavy loads are disconnected and only the control circuits remain powered. This variation is typically unacceptable for electronic circuits in general, and in wireline logging systems in particular. It is thus desirable to maintain a downhole voltage at a fixed value so that tools do not have to cope with changing voltages.
Referring to FIG. 2, a block diagram representing a traditional voltage regulation system 120 is illustrated. Voltage regulation system 120 compares a measured value of the load voltage VL 122 against a set voltage VSET 124 to determine an error signal 126. Error signal 126 is indicative of the difference between VL 122 and VSET 124. Error signal 126 is then typically provided to an error amplification circuit 128. The amplification circuit 128 typically includes an operational amplifier and may employ a proportional, integrating, and/or differentiating circuit depending upon the application. Amplification circuit 128 typically generates a source voltage VS from error signal 126 to maintain VL at the desired level (VSET).
A conventional voltage regulator such as system 120 requires an accurate measurement of the load voltage VL to implement the voltage control at the voltage source. In many applications where the load is located in the proximity of the source voltage, feeding the load voltage back to the regulator presents no significant problem. In a wireline logging application, however, it is logistically impracticable to feed the load voltage back to the source because of the relatively long length of typical wireline cables. Even if the wireline cable were constructed to include a feedback cable to carry the load voltage signal back to the surface, the loss and delay that would characterize the feedback cable would result in a significantly degraded load signal.
To address the above issue, an electrical cable model of the wireline cable can be used. A voltage source signal (generated at the earth surface) is provided to the input of the electrical cable model. The output of the electrical cable model represents an approximation of the measured signal's effect on the load voltage in the actual electrical cable. This approximation can be used to alter the source voltage to cancel out the effect of the cable on the load voltage. An issue associated with using an electrical cable model to compensate for effects of a wireline cable is that, conventionally, efficient and accurate techniques of characterizing the wireline cable are typically not available.